1. Field of the Invention
The present invention relates generally to a hydraulic control system for an automatic transmission for use in automotive vehicles and more specifically to a pressure regulation arrangement which controls the pressure supplied to selected a friction element or elements of the transmission.
2. Description of the Prior Art
The Saab 9000 Service Manual published in 1986, discloses a so called ZF4HP18 type transmission produced by the Swedish Saab-Scania Company describes a transmission which includes a spool and spring biased accumulator piston type pressure control arrangement wherein the spool is used to as a pressure regulator valve. The spool regulates or modifies the pressure in accordance with the bias applied from the spring which is disposed between it and the accumulator piston. As the piston strokes toward the spool the force applied by the spring increases. Accordingly, as the amount of stroke increases the bias produced by the spring increases and induces an increase in the level of the modified pressure. This pressure is supplied to a predetermined friction element of the transmission and, due to the increasing level of the same, induces smooth engagement.
The pressure which is used to control the movement of the spool is produced by a valve (referred to hereinunder as a throttle pressure modifier valve) which is modulates line pressure in accordance with the level of the throttle pressure (viz., a pressure indicative of the torque being applied to the engine transmission). The pressure discharged by this valve is supplied into a chamber of the pressure regulator valve defined between the spool and the accumulator piston.
In brief, with this arrangement as the level of the torque indicative pressure increases, the level of the regulated pressure discharged by the regulator valve is in turn increased. Accordingly, when the level of the torque indicative pressure (throttle pressure) increases in accordance with the operation of the engine, the level of the modified throttle pressure also increases. As a result, the level of the adjusted or regulated pressure also increases and the amount of torque which can be transmitted by the friction elements is increased.
However, with this system it is necessary to provide a special valve (viz., the throttle pressure re-adjusting or modifier valve) between the source of torque indicative pressure (throttle pressure) and the regulator valve in order to adjust the level of the throttle pressure before it is applied to said pressure regulator valve. That is to say, in the event that the throttle pressure modifier valve is not provided and throttle pressure is supplied directly between the spool and the accumulator piston of the regulator valve, the hydraulic fluid which is discharged from the regulator valve chamber defined between the spool and the accumulator piston, enters the throttle pressure circuit and even though the operation of the regulator valve is not effected, the amount of hydraulic fluid which must be drained by way of the throttle valve becomes excessive and causes a brief increase in the throttle pressure level. As the throttle pressure is supplied to shift valves of the transmission, this momentary boost in the level thereof increases the bias with which the spools of the shift valves are urged toward their respective downshift positions and possibility of an undesired downshift is brought in existence. In addition, this momentary fluctuation in the throttle pressure tends to induce the transmission shift point to undergo a sympathetic fluctuation and hunting of the transmission between gear ratios is rendered possible. For this reason it has been hitherto been necessary to provide the throttle pressure modifier valve. However, even when such a valve is provided, while the above mentioned undesirable effect on the throttle pressure is prevented, the amount of hydraulic fluid which can be drained by the throttle pressure modifier valve tends to be inadequate and the regulated pressure tends to be increased as a result.